Heat dissipation from active semiconductor devices has become a major technical challenge in achieving continued increases in performance and function. The thermal resistance between the backside of a flip chip and the heat spreader attached to it decreases as the bulk thermal conductivity of the thermal interface material (TIM) increases and as the bond line thickness decreases. Therefore, accurate measurement of the bond line over the entire chip area is essential to design and predict the thermal resistance. Conventional methods to measure bond line include cross-sectioning; laser profiling before and after lid attach; and lid removal after cure combined with z-scope measurement through the adhesive thickness.
The most common method is cross-sectioning which is both destructive and time consuming providing only a few points of measurement over the chip area. First the module is encapsulated to fill any open cavities in the package. After cure is complete, a diamond saw is used to cut the sample and expose a cross section that could be along an edge or diagonal. This sample piece is then encapsulated again to facilitate the hand grinding and polishing preparation that is required to reveal clean interfaces that allow measurement of the bond line. However, because of the large difference in the hardness of materials that include silicon, copper and polymers, materials are torn out or smeared during grinding and polishing therefore making it difficult to discern the exact interface. After the polishing sequences, the bond line dimension is measured at several points along the section line using an x-y microscope or digital image analysis. It takes several days to complete cross-sectional analysis on a few modules, so this method is viable for development work and failure analysis but not for real-time process characterization or line monitoring.
Z-scope measurements provide quicker turn around compared to cross-sectioning. Measurements are required in several spots to understand bond line variation over the chip area. One method is to dry place a heat spreader onto a chip and laser scan the lid surface. A thermal interface material is then applied to either the chip or heat spreader and the heat spreader is mated to the chip with a force and a dwell time. After the force is released, a second laser scan is made. The z-direction difference between the two scans is a measure of the bond line. The bond line is a difference of between 2000 and 2050 microns for only 50 microns. Typically this method is used to measure wet or uncured bond lines to make sure the process is in control. For processes that require pressure during cure, this method could only be used after cure as laser scanning of the heat spreader is not possible in a force loaded cure fixture.
A second z-scope method that is destructive entails removing the heat spreader after cure. If the thermal interface material debonds very near one of the interfaces, and near full thickness remains at the other surface, material can be scratched away in a small area and a z-focus microscope can be used to measure the bond line in several areas by focusing on the surface of the thermal interface material and then on the substrate to which it is attached. This method is reasonable and quick for bond lines that are relatively thick (>75 microns).
Flip chips attached to organic carriers are known to warp because of the CTE mismatch between silicon and laminate (2.8 vs 12-18 ppm/C). Thus the bond line between the chip and heat spreader is not uniform. Depending on the package, the chip bending ranges between 50 to 100 microns. Measuring the bond line over the entire chip area is not practical via cross sectioning.
In order to meet thermal resistance design targets, bond lines are decreasing to 25 microns and lower. Thus, it is becoming even more difficult to make accurate measurement of the bond line.